As a direct iron-making method for obtaining reduced iron through direct reduction of an iron-oxide containing material, such as iron ore and iron oxides, with a reducing gas, such as a carbonaceous material, there is conventionally known a shaft furnace process represented by the so-called Midrex process. According to that type of direct iron-making method, a reducing gas produced from natural gas, etc. is blown into a shaft furnace through a tuyere formed in a lower portion of the shaft furnace, and metallic iron is obtained by reducing iron oxides with the aid of reducing power of the blown reducing gas. Recently, attention has been focused on a process of producing reduced iron, in which a carbonaceous material, such as coal, is used as a reducing agent instead of natural gas. By way of example, the so-called SL/RN process has been already put into practice.
As another method, U.S. Pat. No. 3,443,931 discloses a process of mixing a carbonaceous material and powdery iron oxides in the form of agglomerates or pellets, and reducing the mixture under heating on a rotary hearth, thereby producing reduced iron.
Further, Japanese Unexamined Patent Application Publication No. 2000-144224 discloses a method of supplying an iron-oxide material containing a carbonaceous material onto a hearth of a rotary hearth furnace, and reducing the iron-oxide material under heating, thereby producing reduced iron. That disclosed method employs a rotary hearth-furnace having the interior, which is made up of a material supply zone (12), burner zones (14, 16), a reaction zone (17), and a discharge zone (18). By employing such a rotary hearth furnace, iron oxides are reduced on the hearth surface kept at high temperature and slag components are separated from produced iron, whereby high-purity iron with a carbon concentration of 1 to 5 mass % can be produced. According to that disclosed method, a highly reducing gas atmosphere is maintained in the vicinity of raw materials during the progress of reduction with the presence of a reducing gas (comprising primarily carbon monoxide) that is generated as a result of the reaction between the carbonaceous material and the iron oxides which are both contained in the raw materials. In the last period of reduction corresponding to the reaction zone (17), however, the amount of the generated reducing gas is reduced and the concentration of an oxidizing gas, such as moisture and carbon dioxide generated as exhaust gas upon burner combustion for heating the raw materials, is relatively increased. This leads to a risk that the reduced iron produced as a product is re-oxidized. Particularly, since there is a variation in the progress of reduction of the iron oxides in the last period of the reduction, reduced iron having progressed more sufficiently in reduction tends to be more easily re-oxidized. In some cases, therefore, that reduced iron is not sufficiently carburized and melted, and then discharged while it remains in a not-yet molten state.
In view of the above-mentioned problems in the related art, an object of the present invention is to establish a technique which is applied to a process of producing metallic iron by reducing raw materials including an iron-oxide containing material and a carbonaceous reducing agent under heating, and which can minimize re-oxidization of the metallic iron, i.e., a problem occurred in the last period of solid-state reduction, and can efficiently produce metallic iron having a high metallization ratio and high iron purity at a high yield.
Another object of the present invention is to establish a technique which can minimize the FeO concentration in molten slag in the last period of solid-state reduction, can suppress erosion of a hearth refractory caused by molten FeO to prolong the life of the hearth refractory, and is suitably practiced for long-term continuous operation while improving maintainability of a plant.